\the\font produces no output

Question

According to this answer,

You can access the current font using the \font command, i.e. using \the\font which expand to e.g. \OT1/cmr/m/n/10 for the normal Computer Modern font.

Then why does the following LaTeX manuscript, when compiled with pdfTeX (TeXLive 2017), produce no pdf file?

\documentclass{article}
\begin{document}
\the\font
\end{documen‌​t}

Answer 1

It turned out that the OP’s preference goes to \expandafter\string\the\font.

The following (compilable) example compares three approaches that have been proposed:

\documentclass[a4paper]{article}
\usepackage[T1]{fontenc}
\begin{document}
Compare
\begin{center}
    \expandafter\meaning\the\font
\end{center}
with
\begin{center}
    \expandafter\string\the\font
\end{center}
and with
\begin{center}
    \fontname\font
\end{center}
\end{document}

The output is:

---

Addendum

Let me add a few remark, just to clarify a couple of points.

As I already observed in a comment, in the so-called New Font Selection Scheme (NFSS), which is part of LaTeX2e, the fact that the control sequence for selecting a font has the form

\<encoding>/<family>/<series>/<shape>/<size>

is not a “happy incident”, but a precise design principle on which the NFSS itself relies heavily.

Simply think of how a typical “low level” font selection like

\fontfamily{cmdh}\fontseries{m}\fontshape{n}%
\selectfont

works: first, the three declarations in the first line set the corresponding internal macros \f@family, \f@series, and \f@shape; then, the \selectfont command uses the updated values to build, and subsequently invoke, the appropriate “font-selecting” control sequence. Indeed, the LaTeX2e kernel defines \fontfamily, \fontseries, and \fontshape as follows:

\DeclareRobustCommand\fontfamily[1]{\edef\f@family{#1}}
\DeclareRobustCommand\fontseries[1]{\edef\f@series{#1}}
\DeclareRobustCommand\fontshape [1]{\edef\f@shape{#1}}

\fontencoding and \fontsize are a bit more complicated, but among the other things they do they set, in a similar way, the internal macros \f@encoding and \f@size. On the other hand, the definition of \selectfont is

\DeclareRobustCommand\selectfont
        {%
    \ifx\f@linespread\baselinestretch \else
      \set@fontsize\baselinestretch\f@size\f@baselineskip \fi
    \xdef\font@name{%
      \csname\curr@fontshape/\f@size\endcsname}%
    \pickup@font
    \font@name
    \size@update
    \enc@update
    }

The lines that are relevant to our discussion are

    \xdef\font@name{%
      \csname\curr@fontshape/\f@size\endcsname}%
    \pickup@font
    \font@name

As you can see, the macro \font@name is made to expand to a control sequence whose name is obtained by the concatenation of the full expansion of \curr@fontshape, a /, and the full expansion of \f@size; since \curr@fontshape is (statically) defined as

\def\curr@fontshape{\f@encoding/\f@family/\f@series/\f@shape}

and since this definition gets fully expanded, we see that this process indeed yields a control sequence of the form

\T1/cmdh/m/n/10

(for example). The \pickup@font macro ensures that this control name is defined and that it loads the correct external font (this is where the real “hard work” takes place!), after which the font selector that has just been constructed is invoked by executing \font@name.

We can add that \size@update and \enc@update are simply “hooks” where actions that need to be executed when the font size and/or encoding, respectively, have changed can be stored, and that the two lines of code that mention \baselinestretch take into account the possibility that the user has directly modified this obsolete LaTeX2.09 style parameter.

All that said, and following a suggestion from the OP, we can supplement our original example with a couple of other methods to obtain information about the current font:

\documentclass[a4paper]{article}
\usepackage[T1]{fontenc}
\begin{document}
Compare
\begin{center}
    \expandafter\meaning\the\font
\end{center}
with
\begin{center}
    \expandafter\string\the\font
\end{center}
and with
\begin{center}
    \fontname\font
\end{center}
Other methods that use internal commands follow.
\makeatletter
Taking font components apart:
\begin{center}
    \begin{tabular}{ll}
        Encoding: & \f@encoding  \
        Family:   & \f@family    \
        Series:   & \f@series    \
        Shape:    & \f@shape     \
        Size:     & \f@size
    \end{tabular}
\end{center}
All in a row:
\begin{center}
    \selectfont % needed in order to get "\font@name" in sync
    \expandafter\string\font@name
\end{center}
\makeatother
\end{document}

The corresponding output is:

Answer 2

The usage of \the\font is to define a control sequence that expands to the current font selector.

So \edef\thisfont{\the\font} would make \thisfont expand to, say, \tenrm.

Used by itself it just produces the font selector, hence no output.

You may also do

\expandafter\let\expandafter\thisfont\the\font

and \thisfont would become equivalent to the font selector for the current font.

Maybe you were meaning \fontname\font, that expands to the name of the TFM file corresponding to the current font.